The results of the previous sections have shown us that
the general results extracted in the past years about
gaugino condensation in string models, in terms of the
field , are robust.
We have seen how gaugino condensation can in principle
lift the string vacuum degeneracy and break supersymmetry
at low energies (modulo the problems mentioned before).
But this is a very particular field theoretical mechanism
and it would be surprising that other nonperturbative effects
at the Planck scale could be completely irrelevant for these
issues. In general we should always consider the two types
of nonperturbative effects:stringy (at the Planck scale) and
field theoretical (like gaugino condensation). Four different scenarios
can be considered depending on which class of mechanism solves
each of the two problems: lifting the vacuum degeneracy and breaking
supersymmetry.

For breaking supersymmetry at low energies, we expect that
a field theoretical effect should be dominant in order
to generate the hierarchy of scales (it is hard to believe that
a nonperturbative effect at the Planck scale could generate
the Weinberg-Salam scale).
We are then left with two preferred scenarios:
either the dominant nonperturbative effects are field theoretical,
solving both problems simultaneously, or there is a `two steps' scenario
in which stringy effects dominate to lift vacuum degeneracy
and field theory effects dominate to break supersymmetry.
The first scenario has been the only one considered so far,
it includes gaugino condensation and also the discussion of the
previous section in terms of field-dependent soft breaking terms.
The main reason this is the only scenario considered so far is that we can control field theoretical
nonperturbative effects but not the stringy. In this scenario,
independent of the particular mechanism, we have to face the cosmological moduli problem.

In the two steps scenario
the dilaton and moduli fields are fixed at high energies
with a mass
thus avoiding the cosmological moduli problem.
It is also reasonable to expect that Planck scale
effects can generate a potential for and .
The problem resides in the implementaion of this scenario
[99,75],
mainly due
to our ignorance of nonperturbative string effects.

In the two steps scenario, after we have fixed the vev
of the moduli by stringy effects, it remains the question of
how supersymmetry is broken at low energies. Notice that we
would be left with the situation present before the advent
of string theory in which the gauge coupling is
field independent. In that
case we know from Witten's index that gaugino condensation cannot
break global supersymmetry. Since there are no `moduli'
fields with large vev's, the supergravity correction should
be negligible because we are working at energies much smaller
than .

In fact we can perform a calculation by
setting to a constant in eq. (41),
it is straightforward to show that supersymmetry
is still unbroken in that case [99], as expected. A more general
way to see this is
computing explicitly the correction to a
global supersymmetric solution , and see that it
coincides with the solution of
which is always a
supersymmetric extremum of
the supergravity scalar potential.

There seems to be however a counterexample in the literature
[74],
where supersymmetry was found to be broken with vanishing cosmological constant in supergravity but unbroken in global supersymmetry.
Nevertheless it can be seen that
in that case, the global limit is such that vanishes, and so the kinetic energy
for . This makes the corresponding
minimum in the global case ill defined, since there may be other
nonconstant field configurations with vanishing energy.
This is then not a counterexample, because the
global theory is not well defined in the minimum.

We are then left with a situation that if global supersymmetry
is unbroken, we cannot break local supersymmetry, unless
there are moduli like fields.
If we insist to have the two steps scenario,
this can bring us further back to the past and reconsider models
with dynamical
breaking of global supersymmetry.
These models have attracted recent attention partly due to the better
understanding of supersymmetric models from Seiberg and collaborators.
Then gravity will no longer be the mediator of supersymmetry breaking.

Independent of string theory we can classify broken supersymmetric
models
by the mediator of supersymmetry breaking. Currently there are three
main
scenarios considered: the standard gravity mediated scenario that we
have discussed in which the supersymmetry breaking scale is of the
order
of GeV, the gauge mediation scenario
[100] in which gauge
interactions instead of gravity mediate the breaking of supersymmetry,
in this case the scale of breaking has to be close to 1 TeV and more
recently,
it was discovered a new universal mechanism for communicating the
breaking of supersymmetry known as anomaly mediation [101], since the
existence of the conformal anomaly is enough to communicate the
breaking of supersymmetry to the observable sector. Each scenario
has its pros and cons and all are under constant consideration in
phenomenological studies. Which scenario will be the dominant in
string theory is very model dependent.

Therefore, there is not yet a compelling
scenario for supersymmetry breaking
and the field remains open, but now we have a much
better perspective on the relevant issues. The
nonrenormalizable hidden sector models of which the
gaugino condensation is
a particular case, may need a convincing solution of the
cosmological moduli problem to still be considered
viable. Hopefully, this will lead to interesting feedback
between cosmology and string theory [102].
A good example of this string-cosmology interaction is
the recent work by the authors of ref.[103], on which
investigations on string cosmology is leading to interesting
experimental searches for gravitational waves in ranges not
explored
before.
Furthermore, the recent progress in understanding
supersymmetric gauge theories can be of much use for
reconsidering gaugino condensation with hidden matter,
the discussion in the string literature is far from complete.
The understanding of models with chiral
matter could also provide new insights to
global supersymmetry breaking, relevant to the
two steps scenario mentioned above.